A solid polymer fuel cell is a power-generating apparatus wherein hydrogen and oxygen are supplied to the fuel electrode and the oxidant electrode, respectively. The fuel electrode and the oxidant electrode are bonded to the respective surfaces of a solid polymer electrolyte membrane containing perfluorosulfonic acid acting as an electrolyte.
The following reactions take place in the respective electrodes.                Fuel Electrode: H2 2H++2e        Oxidant Electrode: ½ O2+2H++2e H2O        
Higher power of 1 A/cm2 or more can be obtained at ambient temperatures and atmospheric pressure in accordance with the reactions in the solid polymer fuel cell.
In the fuel electrode and the oxidant electrode, mixtures consisting of carbon particles supporting catalyst metal and a solid polymer electrolyte are present. Generally, these mixtures are applied on the electrode substrates such as carbon paper acting as layers for diffusing fuel gas. These two electrodes sandwiching therebetween the solid polymer electrolyte membrane are thermally bonded under pressure to configure the fuel cell.
In the fuel cell thus configured, hydrogen gas supplied to the fuel electrode reaches to the catalyst after passing through fine pores in the electrode to be converted into the hydrogen ions by releasing the electrons. The released electrons are introduced to an external circuit after passing through the carbon particles and the solid polymer electrolyte, and flow into the oxidant electrode through the external circuit.
On the other hand, the hydrogen ions generated on the fuel electrode reach the oxidant electrode through the solid polymer electrolyte in the fuel electrode and the solid polymer electrolyte membrane sandwiched between both the electrodes, and then form water by means of the reaction with oxygen supplied to the oxidant electrode and the electrons flowing from the external circuit in accordance with the above reaction formula. As a result, the electrons flow from the fuel electrode toward the oxidant electrode in the external circuit to provide electric power.
In order to improve the characteristics of the fuel cell having the above configuration, a larger adhesive force is important at the interfaces between the electrodes and the solid polymer electrolyte membrane. That is, a higher ionic conductivity of the hydrogen ions generated through the electrode reaction is desired at the interfaces between them. A poor adhesion on the interface increases the electric resistance due to the decrease of the conductivity of the hydrogen ions and results in a reduction of the cell efficiency.
While the fuel cell using the hydrogen as the fuel has been described heretofore, the research and the development regarding a fuel cell using an organic liquid fuel such as methanol have been extensively conducted in recent years.
In the fuel cell using the organic liquid fuel, it is known that the organic liquid fuel is modified to the hydrogen gas for use, or the organic liquid fuel is directly supplied to the fuel electrode as represented by the direct-methanol type fuel cell.
In the fuel cell in which the organic liquid fuel is directly supplied to the fuel electrode, no apparatus such as a reformer is required because the organic liquid fuel is directly supplied to the fuel electrode. Accordingly, the cell structure can be made simpler and the entire apparatus can be advantageously made downsized. Further, the organic liquid fuel has an advantage of being more easily and safely transported than the gas fuel such as the hydrogen gas and hydrocarbon gas.
Generally, in the fuel cell using the organic liquid fuel, the solid polymer electrolyte membrane made of solid polymer ion exchange resin is used as the electrolyte. In order to operate the fuel cell, the hydrogen ion is required to move from the fuel electrode to the oxidant electrode through the above membrane, and the movement of the hydrogen ion is accompanied with the movement of water. Accordingly, the membrane is required to contain a certain amount of moisture.
However, in case of using the organic liquid fuel having the higher hydrophilicity to water such as methanol, a problem to be overcome arises that organic liquid fuel diffuses in the solid polymer electrolyte membrane containing the moisture to further reach to the oxidant electrode (crossover). The crossover brings about the reductions of the voltage, the output and the fuel efficiency because the organic liquid fuel which should essentially supply electrons to the fuel electrode is oxidized on the oxidant electrode so that the organic liquid fuel is not effectively used as the fuel.
In view of overcoming the problem of the crossover, the suitable selection of polymer having lower water content as the material of the solid polymer electrolyte membrane is desirable for suppressing the diffusion of the organic liquid fuel such as the methanol with the water. However, in connection with the catalyst layer on the surface of the electrode which is adjacent to the above electrolyte membrane, it is important to supply a plenty of the hydrogen ions thereto by efficiently moving, from the electrode layer, the organic fuel cell to be used as the fuel. That is, it is desirable that the catalyst layer on the electrode surface allows the organic liquid fuel to better permeate and electrolyte membrane does not allow the organic liquid fuel to permeate. In order to achieve these, it is suitable that a material having a higher water content and a higher permeability regarding the organic liquid fuel is used as the polymer forming the catalyst layer on the electrode surface, and a material having a lower water content and lower permeability regarding the organic liquid fuel is used as the polymer which forms the solid polymer electrolyte membrane.
JP-A-2001-167775 describes a technique regarding a membrane having ionic conductivity in which the crossover of methanol can be suppressed while maintaining the ionic conductivity. In the publication, the surface layer of the ionic conductive membrane having fluorine resin as its main structure, such as Nafion (registered trademark) is modified by the electron beam radiation such that the conductivity of the surface layer is lower than the interior of the membrane.
However, when the materials of the catalyst layer of the electrode surface and the solid polymer electrolyte membrane are different from each other, the sufficient adhesion cannot be obtained and thereby the peel-off may take place at the interface between the electrode surface and the solid polymer electrolyte membrane. The peel-off increases the electric resistance on the interface and cause the reduction of the reliability regarding the cell performance. The modification of the surface layer of the ionic conductive membrane as described in the above JP-A-2001-167775 also has a problem that the surface strength increase at the time of swelling of the ionic conductive membrane worsens the adhesion with the catalyst layer of the electrode surface. In view of these circumstances, an object of the present invention is to increase the adhesion at an interface between an electrode surface and a solid polymer electrolyte membrane, thereby improving cell performances and reliability of a cell.
Another object of the present invention is to suppress crossover of an organic liquid fuel while maintaining excellent hydrogen ion conductivity on the electrode surface and permeability of the organic liquid fuel.